Thermal expansion relates to the tendency of matter to increase in volume or pressure when heated. When materials expand and contract, tensile forces and compressive forces are created, respectively. The amount of expansion varies depending on the material's coefficient of thermal expansion.
In engineering, thermal expansion of materials has to be taken into account for numerous reasons. Though this phenomenon can be beneficial, especially in thermometers and shrink-fitting, it can often present problems. In order to solve this problem, many studies have been conducted in search of controlled thermal expansion.
In 1896, Charles Edouard Guillaume invented the alloy Invar (FeNi). Invar is an alloy of iron (64%) and nickel (36%) with some carbon and chromium. This alloy is known for its unique properties of controlled coefficient of thermal expansion, with different variations in the stoichiometry allowing the alloy to obtain this property.
The Department of Materials Science and Metallurgy at the University of Cambridge confirmed that by considering a wide range of laminated composites containing a variety of fibers and matrices, negative thermal expansion coefficients may be obtained. These are usually accompanied by a correspondingly large value of the in-plane axial Poisson's ratio (PR). Through this large PR, large negative values of thermal expansion may be obtained, much greater than for any monolithic materials. The use of laminated composites also overcomes some of the limitations of a device to control thermal expansion when made with monolithic materials.
These studies have provided some limited solutions to avoid the problem of thermal expansion. However, these solutions require the manipulation of crystalline structures, or the combination of multiple materials, usually composites. Any devices incorporating these solutions may, therefore, be somewhat complicated and difficult to manufacture.
One device for use in temperature related applications, such as in a thermostat, is a bimetallic strip. These bimetallic strips generally include two distinct metals, having different coefficients of thermal expansion, bonded together along their length to form a flat beam. Due to the differing coefficients of thermal expansion, the two materials expand at different rates in response to a temperature change. As a result, the bimetallic strip coils and uncoils in response to a change in temperature, thereby automatically controlling the amount of heat being provided by the thermostat. An example flat bimetallic strip design, as known in the art, is shown in FIG. 1. However, the geometry of these strips (with two materials bonded along their lengths), has generally limited the use of bimetallic strips to thermostat type applications.